Hourly Relative Humidity and Fall Injuries in China: A Nationwide Case-Crossover Study Based on 4.19 Million Cases

Falls represent a significant public health issue worldwide. Previous studies have demonstrated associations of temperature with fall risk, but the effect and burden of hourly relative humidity and fall morbidity have been rarely investigated. Fall data and meteorological data were collected from the National Injury Surveillance System during 2006–2021 in China. Associations between hourly relative humidity and fall onset were analyzed using conditional logistic regression combined with a distributed lag nonlinear model. Stratified analysis was used to identify potential association modifiers. Several sensitivity analyses were applied to examine the robustness of the results. The study involved 4 197 495 fall-related cases. Relative humidity exhibited monotonic relationships with the fall onset risk. The excess risk (ER) of falls appeared immediately at lag 0 h and persisted for 8 h. A 10% decrease in relative humidity was associated with a 1.35% (95% confidence interval [CI]: 0.83–1.87%) increase in excess risk of falls and much higher risk for males (ER = 1.89%, 95% CI: 1.29–2.48%) than that for females (ER = 0.52%, 95% CI: 0.14–0.90%). The excess risk of fall injury was the highest among individuals aged 5–14 years (ER = 3.45%, 95% CI: 2.57–4.32%) compared to other age groups. Moreover, compared to the highest relative humidity, the attributable fraction (AF) due to low relative humidity exposure was 4.94% (95% CI: 4.88–4.98%). Subgroup analyses revealed that males, individuals aged 5–14 years, students, cases occurring in public places, and cases engaged in sports activities were more susceptible to relative humidity than their correspondents. Regional disparities were observed, with individuals in southern China or warm regions showing greater sensitivity to changes in relative humidity. Results of this study suggest that hourly low relative humidity exposure is associated with an increased risk of fall onset. These findings underscore the necessity of time-specific interventions to mitigate the relative humidity risk.